1. Field of the Invention
The present invention relates to structures and methods for producing large, single-crystals of silicon carbide with crystalline quality suitable for use in semiconductor devices.
2. Description of the Prior Art
Silicon carbide is a wide bandgap semiconductor material with physical and chemical properties unmatched for high power microwave, temperature tolerant, and radiation resistant applications. For example, silicon carbide exhibits a critical electric-field breakdown strength of approximately ten times that of silicon, the most commonly used semiconductor material. Also, silicon carbide has a high-field electron velocity equal to gallium arsenide, a semiconductor material commonly utilized for its high electron velocity property. Further, silicon carbide exhibits a higher thermal conductivity (an advantageous property in the operation of certain semiconductor-based devices) than silicon and gallium arsenide, having a value near that of copper. Silicon carbide based microwave transistors and integrated circuits could provide approximately five times the power density of gallium arsenide based MMICs at X-band frequencies, and approximately ten times the power density of silicon at UHF-band to S-band frequencies.
A basic requirement for the use of silicon carbide as a semiconductor material is that the silicon carbide be prepared as large single crystals. The large single crystals should be prepared with a size sufficient for economic applications and have a structure consisting of a repeated atomic arrangement with a minimum of defects in the periodic array.
Single crystals of silicon carbide have been obtained as platelets found within cavities in the large Acheson furnaces used to produce silicon carbide grain. These platelets were found in general to be defective, with no control of the platelet growth process.
The first single crystals were grown in the laboratory by a sublimation vapor-condensation growth technique by J. A. Lely that was reported in Ber. Deut. Keram. Ges., 32, (1955) pp. 229-251 and described in U.S. Pat. No. 2,845,364. In this technique, Lely constructed an artificial cavity with pieces of silicon carbide from the Acheson process, placed the cavity within a graphite crucible and heated this charge to above 2500.degree. C. in an inert argon atmosphere where vapors from the subliming charge condensed at random sites within the cooler cavity wall to form platelets of silicon carbide.
Between 1958 and 1978, a number of investigators expended a large amount of activity in an attempt to control the sublimation-vapor transport process and improve the size and quality of the grown platelets. However, the essential drawbacks to the Lely process--uncontrolled nucleation of multiple intergrown platelets, the small size of platelets, and non-uniform growth rate of individual platelets--were not overcome. In addition, the silicon carbide platelets generally contained layers of different polytype layers with crystallographic structure having different stacking orders of the silicon and carbon atoms comprising the crystal. While the different polytypes exhibit nearly identical physical properties, a significant difference may be observed in the electrical and optical properties of each polytype. Single-crystal, single-polytype silicon carbide material is required for efficient device fabrication, since the occurrence of random polytypes within the crystal will adversely affect the electronic properties of devices fabricated on them.
The growth of cylindrical single-crystal boules was first described in a publication by Yu. M. Tairov and V. F. Tsvetkov, in J. Crystal Growth 43, (1978) pp. 209-212, in which small ingots 8-mm in diameter and 8-mm in length were grown. This development incorporates a seed crystal in order to control nucleation, and in this manner is similar to the growth techniques used to grow single crystals of silicon. In this technique, as described in Yu. M. Tairov and V. F. Tsvetkov, J. Crystal Growth 52 (1981) pp. 146-150, a graphite crucible is used wherein a single crystal seed placed in one portion of the crucible is separated from silicon carbide grain placed in another portion of the crucible. The seed temperature is raised to a temperature of 1800-2000.degree. C. and the source temperature raised to a higher temperature than the seed, and sufficient to provide a temperature gradient of about 30.degree. C./cm between source and seed. Nucleation of growth on the seed surface was effected under approximately a 100 Torr pressure of argon to stabilize the polytypic structure of the deposition. The growth rate was then slowly increased by evacuating the system to a pressure of 10.sup.-1 -10.sup.-4 Torr according to an exponential law with a time constant of approximately 7 minutes. Mass transfer is effected by the fluxes of the gas phase components Si, Si.sub.2 C and SiC.sub.2 formed as a result of decomposition of the SiC source material.
Tairov and Tsvetkov further noted that the vapor pressure of Si exceeds the vapor pressure of Si.sub.2 C and SiC.sub.2, and reacts with the lower temperature graphite walls of the growth cell. Growth at source-to-seed separations of greater than 10 mm was obtained as silicon vapor acts as a carbon transporting agent with the graphite cell being the carbon source. Tairov and Tsvetkov suggested that it is possible to control the polytypic structure of the growth by selecting seeds of the required polytype, or by growing the crystal on faces forming an angle to the (0001) surface. To grow polytypic homogeneous crystals, it is also necessary to eliminate supersaturation fluctuations during crystal growth.
The production of 6H-polytype single crystal boules up to 20-mm in diameter and 24-mm in length suitable for use as blue light emitting diode substrate material was reported by Ziegler et al., in IEEE Trans. Electron Dev., ED-30, 4 (1983) pp. 277-281, and described in German patent DE 3,230,727. Ziegler et al. referred to this process as the "modified Lely technique". The process described by Ziegler et al. is based on the knowledge that the Tairov method produced crystals with mixed polytypes due to the following: the temperature gradient was too high, the pressure of the protective gas was too low, and the temperature of the seed should be selected in accord with the vapor pressure diagram given by Knippenberg in Growth Phenomena in Silicon Carbide, Philips Research Reports 18, (1963) pp. 164-166.
The process described by Ziegler et al. limits the temperature gradient in the direction of epitaxial growth to no more than 25.degree. C./cm, holds the temperature of the seed crystal to a range of 2100-2300.degree. C., and adjusts the pressure of the protective gas to be at least as high as the total of the gas partial pressures of the deposition components. Ziegler et al. described a proportedly advantageous arrangement of the sublimation growth crucible by placing porous graphite outside the reaction zone and placing the sublimation source behind the porous partition above the deposition zone. The temperature gradient is provided by additional heating of the source end of the crucible, or by additional cooling of the seed. The cooling of the seed is typically achieved by conductive cooling means such as a "cooling finger" or "cold finger" extending out of the crucible into the vacuum chamber. The use of a cold finger is familiar to those with ordinary expertise in the area of condensation of vapors. And, while the use of a "cold finger" is normally restricted to the condensation of vapor to form a liquid, the cooling of a vapor to form a solid will be familiar to those practiced in the art of crystal growth.
U.S. Pat. No. 4,886,005 to Davis et al. provides a method of reproducibly controlling the growth of large single crystals of silicon carbide using a technique called physical vapor transport "PVT"), a technique also referred to as the "modified Lely technique". Physical vapor transport is the current preferred general method in the industry for the growth of silicon carbide crystals, and consists of a furnace having a graphite (carbon) crucible with a cavity therein. The furnace further has a means for heating the crucible and cavity. A source material of silicon carbide is provided at a first region within the crucible cavity, while a silicon carbide monocrystalline seed is positioned at a second region within the crucible cavity. In silicon carbide crystal growth, where the sublimation space must be kept above 1900.degree. C. and due to the high reactivity of the silicon-containing vapor, carbon or graphite are currently the only crucible materials capable of practical use. The presence of a free carbon is an important part of the chemical reaction to produce silicon carbide crystalline material, where the principal vapor components are Si, Si.sub.2 C and SiC.sub.2 (Drowart et al., J. Chem. Phys., 29 pp. 1015-1021, 1958).
According to Davis et al., controlled, repeatable growth of silicon carbide of desired polytype is achieved by generating and maintaining a substantially constant flow of vaporized Si, Si.sub.2 C, and SiC.sub.2 per unit area per unit time from the source to the growth surface of the seed crystal. To maintain this constant flow of vaporized Si, Si.sub.2 C and SiC.sub.2, a source powder is used having a selected graph size distribution and that has substantially the same desired polytype as the seed crystal. Further, the source is fed by various means to maintain essentially constant source characteristics. Davis et al. also describe the use of a monocrystalline seed of desired polytype with off-axis growth face as previously described by Tairov, and manipulation of the source temperature to maintain a constant temperature gradient between the subliming source powder and the growth surface as would be obvious to those with ordinary experience in crystal growth.
The use of thermal gradients in the modified Lely technique for growth of silicon carbide has been described by Tairov et al. in J. Crystal Growth 52, 1981, p. 147 and by Davis et al. in U.S. Pat. No. 4,866,005. However, if portions of the crucible surrounding the seed crystal are sufficiently cooled while achieving the thermal gradient, nucleation will result. For example, if axial movement of a heating element (such as an induction coil) is used to provide a temperature difference between the source and seed end of the crucible, portions of the crucible may simultaneously be cooled sufficiently to allow nucleation on portions of the graphite crucible surrounding the seed crystal. Such nucleation and incursion of extraneous polycrystalline material into the grown crystal has been demonstrated by Ziegler et al., (IEEE Trans. Electron Dev. ED-30, 4, 1983, p. 279).
Furthermore, if during seed growth, the source is removed from the crystal growth front a distance of more than approximately 10 mm, the higher partial vapor pressure of silicon reacts with carbon in the vicinity of the growth front to provide the deposition species. However, portions of the carbon crucible in proximity to the seed and growth front will become overcooled and will become a surface for nucleation and for the growth of unwanted polycrystalline material.
Thus, a method is needed that allows for the growth of a single crystal of silicon carbide, while preventing the nucleation of extraneous crystallites.